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Separation of Ultrafine Pyrite from High Sulfur Coals by Selective Dispersion and Flocculation YosryA.Attia 148 Fontana Laboratory, Metallurgical Engineering Department and Mining Engineering Division, The Ohio State University, Columbus, OH 43210 A novel technique f o r separating u l t r a f i n e p y r i t e part i c l e s (minus 40 micrometers) from coal fines has been conceptually developed and tested. The technique i n v o l ves the use of a selective polymeric dispersant f o r p y r i t e , while f l o c c u l a t i n g coal p a r t i c l e s with a polymeric f l o c c u l a n t . The suspended p y r i t e can then be removed from the flocculated coal fines which s e t t l e p r e f e r e n t i a l l y by gravity. The key to t h i s separation was the design and preparat i o n of the selective dispersant f o r p y r i t e (PAAX). This was achieved "by incorporating xanthate groups into the structure o f a low molecular weight a c r y l i c acid polymer (PAA). Testing t h i s reagent on i n d i v i d u a l suspensions of coal and p y r i t e confirmed its selective dispersion action towards p y r i t e , while it had no d i s persion action on coal suspension when using P u r i f l o c A22 flocculant. Preliminary tests on high sulfur coals from Kentucky No. 9 seam, Meigs Creek, and Ohio No. 6 also confirmed t h i s s e l e c t i v i t y . Further process development is needed, however, before t h i s promising technique can become commercially f e a s i b l e . Most of the Eastern U.S. coals suffer from high sulfur content, which on burning emit sulfur dioxide in excess of the EPA l i m i t of 1 . 2 l b SO2/million Btu. S p e c i f i c a l l y , there are three broad regions that suffer from severe s u l f u r problems. These are: Northern Appalachia (Pennsylvania, Ohio, West V i r g i n i a ) with average sulfur content of about 3%; the Midwest region (Indiana, I l l i n o i s , Kentucky) with average sulfur of 3.9%> and Central Midwest (Iowa, Missouri, Kansas, Oklahoma) with average sulfur of 5 . 2 5 $ . The presence of sulfur in coal is generally attributed t o two forms, organic sulfur and p y r i t i c sulfur. The proportion of p y r i t i c s u l f u r t o organic s u l f u r varies s i g n i f i c a n t l y from one coal seam t o another, but it appears that p y r i t i c sulfur generally represents about TO percent or more o f the t o t a l sulfur (l). The p y r i t e is
0097-6156/ 86/ 0319-0021 $06.00/ 0 © 1986 American Chemical Society
Markuszewski and Blaustein; Fossil Fuels Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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FOSSIL FUELS UTILIZATION: ENVIRONMENTAL CONCERNS
found in coal in a wide size d i s t r i b u t i o n , with a s i g n i f i c a n t proportion in the very fine size f r a c t i o n (less than 2 5 micrometers). For example, the mean p a r t i c l e size of p y r i t e in the Midwest region is about 37ym (kOO mesh), f o r the Central to Western Midwest region is about 107pm ( 1 5 0 mesh), and in the Appalachian regions is about 6 8 - 1 0 0 y m . This can only mean that a s i g n i f i c a n t amount of the p y r i t e is in the very f i n e to c o l l o i d a l f r a c t i o n s . In f a c t , there are coal seams, such as Kentucky No. 9 , where almost a l l of the p y r i t e part i c l e s are often smaller than about ten micrometers {2). It is believed, therefore, that the separation-removal of pyrite from coal p r i o r t o its combustion would greatly reduce the s u l f u r dioxide emission and render many coal deposits within the new source performance standards. For separation of coal from r e l a t i v e l y coarse shale and p y r i t e , gravity-based techniques have been e f f e c t i v e l y u t i l i z e d . For sizes below 300 micrometers to about 1 0 0 micrometers, f r o t h f l o t a t i o n has been used s a t i s f a c t o r i l y for separating coal from shales (3.). Even separation of p y r i t e from coal has been achieved by f l o t a t i o n (k). However, most of these processes become less e f f e c t i v e when the p a r t i c l e size of the coal suspension is s i g n i f i c a n t l y below 1 0 0 micrometers. One of the promising new technologies f o r separation of very fine p a r t i c l e s is s e l e c t i v e f l o c c u l a t i o n . The s e l e c t i v e f l o c c u l a t i o n process has been used e f f e c t i v e l y to separate very f i n e l y d i s seminated minerals from mixed ore suspensions (5.). The process is based on the p r e f e r e n t i a l adsorption of an organic flocculant on the wanted minerals, thereby f l o c c u l a t i n g them, while leaving the remainder of the suspension p a r t i c l e s dispersed. The dispersion of c e r t a i n components in the suspension such as p y r i t e can be enhanced by using more selective or powerful dispersants. Methods f o r achieving s e l e c t i v e f l o c c u l a t i o n and dispersion have been recently described by A t t i a (6). The objective of t h i s research was to investigate the f e a s i b i l i t y of separating coal fines from mixed suspensions with p y r i t e by s e l e c t i v e f l o c c u l a t i o n . In t h i s work, the separation of p y r i t e from coal was based on the d i s t i n c t differences in t h e i r surface chemical properties. Pyrite being a s u l f i d e mineral has an a f f i n i t y for xanthate-containing reagents, while coal does not have such an a f f i n i t y towards xanthates. Therefore, i f xanthate groups could be incorporated into the structure of long-chain polymers, selective polyxanthate flocculants or dispersants f o r p y r i t e might thus be achieved. The use of polyxanthates f o r selective f l o c c u l a t i o n of s u l f i d e minerals, p a r t i c u l a r l y f o r copper, was f i r s t reported by A t t i a and Kitchener ( ] ) . The separation of p y r i t e from coal using polyxanthate dispersants was i n i t i a l l y reported by A t t i a and Fuerstenau (8) and more recently described by A t t i a ( 9 . ) . Preparation and Characterization of Polyxanthate Dispersants As reported e a r l i e r (0), polyxanthate dispersants can be prepared by reacting low molecular weight p o l y a c r y l i c acid solutions with sodium hydroxide and carbon d i s u l f i d e . Thus, to a 50-ml 1 percent solution of PAA supplied by S c i e n t i f i c Polymer Products, Inc. (mol. wt. = 2 0 0 0 ) , 5.Ug NaOH p e l l e t s were added and dissolved. Then 15 ml
Markuszewski and Blaustein; Fossil Fuels Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
2. ATT1A
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Separation of Ultrafine Pyrite from High Sulfur Coak
CS2 was added, and the solution was shaken in a water hath at 55°C for 2 . 5 hours. The crude reaction product was carrot-red in color and was referred t o as PAAX. P u r i f i c a t i o n of the reaction products of PAAX was made by mixing 50 ml of the carrot-red solution with excess amounts of methanol (up to 3 0 0 ml) to produce a pale yellow p r e c i p i t a t e . After standing f o r a period of time, the suspension was f i l t e r e d o f f to recover t h i s p r e c i p i t a t e . The p u r i f i e d PAAX p r e c i p i t a t e was r e a d i l y soluble in d i s t i l l e d water. Great care was exercised to minimize decomposition of the polyxanthate during the p u r i f i c a t i o n process. This was p a r t l y achieved by maintaining the solution pH highly alkaline. Characterization of p u r i f i e d PAAX solutions was made by uv v i s i b l e spectroscopy technique using a Model Carey-1T Spectrometer (Varian). The uv spectra of the p u r i f i e d and unpurified reaction products of the xanthated p o l y a c r y l i c acid are shown in Figure 1 . Table 1 shows the molar e x t i n c t i o n c o e f f i c i e n t s f o r the reaction products of the xanthated c e l l u l o s e (viscose) f o r comparision ( 1 0 ) . From the uv spectra and Table 1 , it was concluded that the xanthate groups were present in both the p u r i f i e d and the crude PAAX products. It was also clear that the p u r i f i c a t i o n method used rejected most of the secondary reaction products (the p o l y s u l f i d e s ) from the xanthated product. Most of the secondary reaction products mentioned in Table 1 are present in the uv spectrum of the crude product of PAAX in Figure 1 , but not in the p u r i f i e d product. However, it seemed that the xanthate absorption peaks were s l i g h t l y s h i f t e d t o the higher wavelengths of 3 0 5 to 3 0 8 y m , as compared with 303um f o r most lower molecular weight xanthates. It is important to note that the unmodified p o l y a c r y l i c acid s o l u t i o n , although not shown here, d i d not exhibit absorption peaks in t h i s region of wavelength ( i . e . , 300 to 308ym). Table 1 .
Molar Extinction C o e f f i c i e n t s of Viscose Compounds ( 1 0 )
Wavelength Compounds Sodium c e l l u l o s e xanthate Sodium c e l l u l o s e dixanthogen Sodium d i t h i o carbonate Sodium t r i t h i o carbonate Sodium s u l f i d e Carbon d i s u l f i d e
336ym
303um
250um
226um
17508
10928
6862
12396
600 18200
272um
10500
206um
3250
3hk0 I69O
12230 7730 60,00070,000
Xanthates are usually formed by reacting an alcohol or a raw material containing alcoholic hydroxyl groups with an a l k a l i and carbon d i s u l f i d e ( 1 0 ) . However, in the case of the a c r y l i c acid polymer used here, it is not clear how t h i s reaction would proceed. But whatever the mechanism, it appears from the uv spectra and from
Markuszewski and Blaustein; Fossil Fuels Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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FOSSIL FUELS UTILIZATION: ENVIRONMENTAL CONCERNS
the f l o e c u l a t i o n t e s t s that the p o l y a c r y l i c acid was modified t o contain xanthate groups in its structure. A t t i a (9.) has recently proposed the possible structure of t h i s p o l y a c r y l i c acid/xanthate (PAAX) dispersant as sodium polyacrylate-acrylodithiocarbonate. He also pointed out that the r a t i o of acrylate t o xanthate groups in the polymeric structure depends primarily on the degree of xanthation. Testing of PAAX on Individual Coal and P y r i t e
Suspensions
Selective f l o c c u l a t i o n of coal from p y r i t e vas investigated using the newly developed polyxanthate dispersant in conjunction with purifloc-A22 flocculant (a polystyrene sulfonate). Thus, 50 ml mineral suspensions of each of the coal and p y r i t e containing 2 percent solids of less than 37ym (^00 mesh) p a r t i c l e size were treated with c e r t a i n amounts of the PAAX dispersant ( 5 0 t o hOO mg/l), while s t i r r i n g continued f o r 5 minutes at high shear rate. The i n i t i a l suspension pH before the addition of dispersant was about 10. The f i n a l suspension pH a f t e r the addition of the crude solu t i o n o f PAAX was sometimes higher due t o its high sodium hydroxide content and ranged between pH 10.Τ and 1 1 . 5 · At the end of disper sant conditioning time ( 5 minutes), k mg/1 Purifloc-A22 was admini stered into the suspension, and s t i r r i n g was continued for 10 seconds at high shear, then lowered t o a gentle a g i t a t i o n for an additional. 1 0 seconds. The suspension was transferred into a 100-ml graduated cylinder and was allowed t o stand s t i l l f o r 5 minutes. At the end of the s e t t l i n g period, the suspended s o l i d s were de canted, and the s e t t l e d s o l i d s were recovered. Each f r a c t i o n was placed in an evaporating dish, oven dried and weighed. Selective f l o c c u l a t i o n of coal mixtures with p y r i t e was made on suspensions containing equal proportions of coal and p y r i t e , using 200 mg/l PAAX dispersant at pH 10. The f l o c c u l a t i o n procedure was the same as described above, except that the products were q u a l i t a t i v e l y analyzed by v i s u a l inspection of both f r a c t i o n s . The coal samples used in these experiments were anthracite c o a l , supplied by WilkesBarre, Pennsylvania, and the p y r i t e used was pure crystals from Wards Natural Sciences, Inc., Rochester, N.Y. The f l o c c u l a t i o n results on the i n d i v i d u a l mineral suspensions are shown in Figure 2 (A & B). These graphs show the effect of p o l y a c r y l i c acid dispersant before (PAA) Figure 2A, and after xanthation (PAAX) Figure 2B, on the flocculation-dispersion behavior of i n d i v i d u a l suspensions of coal and p y r i t e with Purifloc-A22 flocculant. From Figure 2(A), it appeared that PAA i n h i b i t e d or restrained the f l o c c u l a t i o n action of Purifloc-A22 on both coal and p y r i t e suspensions at PAA concentrations of 100 mg/l and above. The d i s persive action of PAA in t h i s case was therefore unselective. How ever, the PAAX crude reaction product in Figure 2(B) only dispersed the p y r i t e suspension to the same l e v e l as PAA, while the coal sus pension was t o t a l l y flocculated even at high PAAX concentrations. The polyxanthate dispersant, rather than improving the dispersion of p y r i t e , simply did not adsorb on the coal p a r t i c l e s , thereby creating a selective dispersion action f o r the p y r i t e . These obser vations in Figure 2(B) were repeated and noted several times, even with p u r i f i e d PAAX solutions. Selective dispersion of p y r i t e or
Markuszewski and Blaustein; Fossil Fuels Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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ATTIA
Separation of Ultrafine Pyrite from High Sulfur Coals
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